Heres a trick when battling a legend bring a poke with run away weaken it to red then run away then track it down and its easyily catght with quick balls.
WARNING:some legendarys dont appear in wild after you have run away always save before
Thats a basic, even those little 8 year old players would know that.
Ummm ok although lots of us already knew this.
Try getting a crobat/golbat to level 60+. Use Carbos to max out its speed and make sure it learns Mean Look. Enter the battle with the legendary, use Mean look then whittle down its health. Make sure your Crobat does'nt get knocked out, or the Mean Look effect ends, and the legendary flees.
Also, try this at night, and use dusk balls, which give up to a 4times capture rate (as far as i'm aware).
Happy Catching
Even before that, you could use FEAR to get it to 1 HP.